1. Field of the Invention
The present invention relates in general to a sealing apparatus for providing seal between two rotary members which are arranged coaxially and rotatably relative to each other. In particular, the invention concerns an annular oil seal of outer rubber periphery type provided with a main sealing lip and an auxiliary sealing lip. More particularly, the invention is directed to an improvement of the oil seal in which a vacuum (negative pressure) produced in an annular space defined between the main sealing lip and the auxiliary sealing lip upon relative rotation of the two members is compensated to thereby prevent areal contact pressure of the sealing lips from being excessively increased.
2. Description of the Prior Art
For having a better understanding of the present invention, description will first be made on hitherto known sealing apparatus of the type mentioned above. FIG. 1 of the accompanying drawings shows a prior art annular oil seal of the outer rubber periphery type. Referring to the figure, the annular oil seal generally denoted by a reference numeral 1' includes an outer peripheral portion 11' at which the oil seal 1' is secured to a housing 2', a supporting portion 12' extending radially inwardly from the outer peripheral portion 11', a base portion 14' formed integrally with the supporting portion 12', and a main lip 15' and an auxiliary lip 18' both formed integrally with the base portion 14' and extending inwardly to a rotatable shaft 3' in a bifurcated manner with inclination relative thereto. Formed between the main lip 15' and the auxiliary lip 18' is an annular channel which defines an annular chamber 17' in cooperation with the rotatable shaft 3', when the oil seal 1' is mounted on the latter.
In the structure of the oil seal described above, it is known from experience that a fluid (e.g. air) confined within the annular chamber 17' tends to leak therefrom in the axially inward direction (toward the right as viewed in FIG. 1) under suction exerted onto the main lip 15', when the shaft 3' is rotated, notwithstanding such arrangement that the sealing tip of the main seal lip 15' is in hermetical contact with the shaft 3'. On the other hand, flowing of fluid or air into the annular chamber 17' from the atmosphere is positively prevented under the sealing action of the auxiliary sealing lip 18'. Under the circumstance, the pressure within the annular chamber 17' is lowered, i.e. a vacuum prevails within the chamber 17' (this phenomenon of generation of vacuum in the annular chamber 17' has heretofore been widely known from experience). As the consequence, the main lip 15' and the auxiliary lip 18' made of an elastic material such as rubber or the like undergo elastic deformation in the direction to decrease the volume of the annular chamber for compensating the reduction in pressure, resulting in that the contact pressure of both the lips 15' and 18' which are forcibly pressed against the peripheral surface of the rotating shaft 3 becomes excessively increased, involving increased friction and hence heat generation as well as excessive abrasion. In other words, the oil seal of the structure exemplified in FIG. 1 suffers a serious problem in respect to the sealing performance.
As an attempt to deal with the above problem, i.e. to compenate the vacuum produced in the annular chamber 17', there has also been proposed a structure of the oil seal shown in FIG. 2 of the accompanying drawings. This oil seal differs from the one shown in FIG. 1 in that at least a through-hole 180' is formed in the auxiliary sealing lip 18' to allow the annular chamber 17' to be communicated to the atmosphere so that the normal pressure state is constantly maintained within the annular chamber 17'. However, the oil seal of this structure is also disadvantageous in that dust or like foreign particles will flow into the annular chamber 17' through the constantly opened through-hole 180' and tend to be deposited particularly at the sealing end face of the main lip 15', to thereby bring about premature abrasion of the slideably contacting portions.